agent: Remove thread-auto-capture feature (#36474)

We never ended up using this in practice (the feature flag is not
enabled for anyone, not even staff)

Release Notes:

- N/A

crates/agent/Cargo.toml

@@ -31,7 +31,6 @@ collections.workspace = true
 component.workspace = true
 context_server.workspace = true
 convert_case.workspace = true
-feature_flags.workspace = true
 fs.workspace = true
 futures.workspace = true
 git.workspace = true